1. Field of the Invention
This invention concerns the improvement of a fundus camera which can take photographs other than by fluorescence, by fluorescence under visible light, and by fluorescence under infra-red light.
2. Description of the Prior Art
Conventionally, a fundus camera is known which can take photographs other than by fluorescence, by fluorescence under visible light, and by fluorescence under infra-red light. In this fundus camera, when taking photographs by fluorescence after completing observations to determine the part to be photographed, a photographic switch is operated. Due to the operation of this switch, an exciter filter for fluorescent photography is inserted in the optical path of an illuminating optical system before a flash tube operates. When the flash tube operates, the fundus of the subject's eye is irradiated by illuminating light of specific wavelengths. The subject has previously been given an injection of a fluorescent agent. After a certain time has elapsed, blood containing this fluorescent agent flows into the blood vessels of the subject's eye, and emits fluorescence under the illuminating light of specific wavelengths. A barrier filter for fluorescent photography is also inserted in the photographic optical system in synchronism with the exciter filter. Fluorescence from the fundus is then guided through this barrier filter to a photographic means, and a satisfactory image of the fundus is thus obtained.
However, the concentration of fluorescent agent in the blood is low immediately before it flows into the blood vessels of the fundus. Subsequently it rises, and then falls as blood flows through the small vessels. As a result, the brightness of the fluorescence increases in a first stage, remains constant in a middle stage, and decreases in a later stage. While the barrier filter for fluorescent photography is inserted in the photographic optical system when making fluorescent observations in the later stage, the image of the subject's eye is dark, and it is impossible to carry out proper alignment and focusing of the system. When making observations in the fluorescent photographic mode, therefore, the barrier filter is removed from the photographic optical system if the image of the eye is dark in order to carry out proper focusing and alignment. The photographic switch is then operated, the barrier filter for fluorescent photography is inserted in the photographic optical system in synchronism with, for example, the action of a shutter, the flash tube is operated, and a photograph by fluorescence is taken.
In this fundus camera, however, no provision is made for the change in the length of the optical path when the barrier filter is inserted or removed from the photographic optical system. Even if, a properly focused image of the fundus is obtained when making observations, therefore, it is impossible to obtain a properly focused photographic image.